In recent years, rapid technological advances in microelectronics, nanotechnology and MEMS technology have spurred new types of micro devices and sensors, such as RFID, micro cameras, accelerometers, miniature implantable devices, and micro chemical sensors, playing important roles in industrial automation, military applications, homeland security, environmental monitoring, and biomedicine. These technological advances have also resulted in significant impacts on people's daily lives. For example, mobile electronic devices, such as the laptop computer, cell phone, and personal media player, have become an inseparable part of many people. All these electronic devices rely on electrical energy to power their circuits, and most of them require a communication channel to exchange information with certain host devices, computers or systems. Currently, batteries and wireless technologies are utilized for these purposes. However, in many cases, these solutions are inadequate. For example, running out of battery power in a laptop or a cell phone when a recharging procedure is missed is an unpleasant, but common event. It would be highly desirable if, when a laptop, cell phone, media player or other electronic device is located within a “hot spot”, a wireless router will not only transmit/receive information, but also recharge these devices. With such a technology, these personal devices will not need manual recharging, and their batteries can be made smaller since they are recharged more frequently. Such a wireless energy transfer technology could also be used in other consumer and industrial applications, such as transferring power from a solar panel outside a residential house to the inside without a cable through the construction wall or roof, powering devices or systems inside a sealed, pressured, or vacuum container of either air or liquid, powering and guiding a robot or a vehicle by a series of thin energy cells under the floor or paved road, recharging an electric car by a low-profile wireless charger “mat” on the garage floor, or transferring solar energy to the inside of a parked car to power ventilation fans in order to keep the inside temperature from rising too high, to name just a few.
In the medical field, microelectronic devices can be implanted within the human body to perform a variety of therapeutic, prosthetic, and diagnostic functions. The deep brain stimulation (DBS) device, for example, is used as a brain implant for treating Parkinson's disease and essential tremor. Currently, a surgical procedure is required to replace the entire device when its battery power is depleted. The combined cost for this procedure is approximately $25,000, which has been described as “the world's most expensive battery change.” Wireless energy transfer technology can eliminate the need for these costly replacements.
One particular medical problem of high interest is the design of a wireless network of devices for the human body. Current and future wireless sensors will be able to be patched on the skin or the underside of clothes to perform a variety of important tasks, such as monitoring vital signs and levels of physical activity. Microsensors may also be implanted by either surgery or injection into the inside of the body to perform additional tasks, such as restoring lost vision, hearing, and motor functions, releasing drugs, and monitoring cancer or cardiovascular diseases. In the military, a body sensor network embedded within the clothes is highly desirable since it can potentially produce warning signals of imminent attacks, detect the presence of people or objects of interest, monitor chemicals in the air, evaluate wounds, and communicate with a central station or an assistive device such as a rescue robot.
While the high significance of such a body network has been recognized by both the research community and industry, the problem of providing power and communication functions to a highly distributed network of electronic devices without wired connections and batteries has not been solved.